Offset prevention means for printing presses are in general use and are disclosed in, e.g., U.S. Pat. Nos. 3,550,552 and 4,332,198. Such means customarily includes a spray assembly that dispenses antioffset material, usually in powder form, upon the freshly printed sheets passing from the impression roll to the stacking station of a printing press. The offset prevention means may further include an electrical control system for regulating the cycles of material-spraying operation of the spray assembly such that the material will be dispensed only when it will impact upon a freshly printed sheet, and not when it would pass between successive sheets. A deficiency of the existing control systems is that this desired result will automatically ensue only for as long as the press speed remains constant. If the press speed is changed the control system must be manually readjusted. In addition to being time consuming, such readjustment may be overlooked by the press operator, in which event antioffset material will not be sprayed upon all desired sheet portions and/or will be sprayed into the spaces between adjacent sheets. Either of these results is highly undesirable.